The present invention is related generally to housings for electronic circuitry, and more particularly to a housing for a thick film engine ignition circuit for an automobile or other vehicle.
With the advent of solid-state electronics, the use of thick film solid state circuits has become more commonplace in providing electronic controls that monitor and control the operation of an engine. Because of the need for proximity to the engine, these circuits operate in the engine compartment and are subject to harsh environments including thermal and mechanical shock.
As a consequence, the thick film circuits are typically enclosed in a plastic housing that contains a molded-in metal insert, or lead frame, that makes contact, through wire bonding, to the thick film circuit. It has been the plastic molding industry's practice to manufacture this housing by supporting the lead frame at various points by extending cores from one-half of the mold to matching cores at the other half of the mold, thereby supporting the lead frame between these cores. This procedure was designed to "lock" the lead frame in the center of the cavity prior to injecting the plastic. Once the mold was closed, the lead frame was "locked" and the hot plastic flowed over and around the frame, filling the mold and trapping the frame in the molding.
Further, to reduce costs, it has been industry practice to double up those portions of the lead frame which make contact with the external connector, thereby doubling the effective thickness of the stamping. This practice allowed a very thin stamping for the electrical paths and saved a substantial amount of money in the material used and in the cost of the stamping die.
A problem, however, existed in the portion of the lead frame which was not entrapped between the opposing cores. The unsupported thin metal contact strips became subject to a varying kinetic stress due to the fluid dynamics of the injected, hot-melted plastic which varied greatly from shot to shot due to the constantly changing viscosity during the injection cycle, thereby making the flow unpredictable at any given point over any given time segment. The varying residual stress cracking of the lead frame was due to the varying flexure the lead frame goes through during the injection process, causing metal fatigue at varying points along the lead frame. On occasion, this varying flexure created thin, hairline fractures that separated the lead frame. As the plastic solidified, it shrank and drew the lead frame together, enabling the insert molded module to pass stringent electrical high-pot tests, and to appear as an acceptable, non-fractured device.
Unfortunately, in use catastrophic failures did occur, opening an electric connection and rendering at least a portion, and possibly all of the electronic control circuit, and possibly the engine itself, inoperative. Thus, the extremely harsh engine vehicle environment in which thick film ignition circuitry is utilized has placed stringent structural requirements on the module housing containing that circuitry.
The purpose of this invention is to provide a module construction applicable to an ignition module incorporating a lead frame composed of metal contact strips less than 0.020 inches in thickness and which eliminates any movement of the lead frame which would cause stress thereon during the molding operation.
The invention calls for a circuit housing which includes a pre-mold base which supports the lead frame during a molding operation resulting in encapsulation of the pre-mold base and lead frame. The lead frame is supported on the pre-mold base and the entire assembly is encapsulated thereby trapping the lead frame solidly to the pre-mold base with assured centering of the same within the housing. To prevent the lead frame from "floating" during the molding process, the injection gates in the second process are preferably located above the lead frame and the pre-molded base subassembly, whereby the surface injection pressure of the injected plastic is always greater at the injection side of the cavity than at the bottom side, thereby maintaining a constant higher pressure above the lead frame than below it. This allows the frame to be firmly secured over its entire pre-mold surface instead of at "points", thereby preventing flexure from taking place during formation of the housing during the molding process.
The result is that flexural variance in the lead frame during the injection process is eliminated, thereby eliminating induced stress and subsequent lead frame failure.
Accordingly, it is an object of the present invention to provide an improved construction for an engine ignition module.
It is a more specific object of the present invention to provide an ignition module housing providing improved protection against thermal and vibration-induced stress.
It is another object of the present invention to provide construction an ignition module wherein the lead frame of the module is not subjected to molding stresses during formation of said module.